Installing Windows From CD Boot

When you are installing a new windows after booting from a CD, when it asks you how big in terms of bytes you want it to be, is the number originally there, the biggest it can be (my number was something like 3000000 bytes)? Also, when I look at my C drive in my compter, it says 142 GB free of of 298, which means it totals to be 340 GB, but the box the laptop came with says it has 320 GB total...could this have to do with the confusion about how many MB is in a GB? Because some people say 1024 MB in a GB and so forth. Also I notice on my dell dimension e310 There is this bar blocking a space for a driver, you can see the one for floppy disc on there, but that is blocked my a bar as well. How do you remove them, can it be done only from the inside?